1. Field of the Invention
The present invention relates to an optical passive element, and more particularly, to an amplitude mask and an apparatus and method for manufacturing a long period grating filter using the same.
2. Description of the Related Art
With the recent developments in optical communications, a long period grating filter used as an optical passive element is attracting much attention. The long period grating filter couples a core mode in which light travels through the core of an optical fiber to a cladding mode, and is manufactured by periodically changing the refractive index of the core of an optical fiber sensitive to ultraviolet rays. That is, the refractive index of a portion exposed to light increases, and that of a non-exposed portion does not change, thus a periodic change in refractive index is generated. In order to couple the core mode to the cladding mode, the following Equation 1 must be satisfied: ##EQU1##
wherein .beta..sub.co is the propagation constant of the core mode, .beta.hd cl.sup.n is the propagation constant of an n-th order cladding mode, and .LAMBDA. is a grating period.
When 2.pi.m/.lambda. (here, n is a refractive index) is substituted for .beta. in Equation 1, Equation 1 becomes n.sub.co -n.sub.cl =.lambda./.LAMBDA.. Accordingly, the period .LAMBDA. and the refractive index difference (n.sub.co -n.sub.cl) must be determined to couple a certain wavelength to the cladding mode. The refractive index difference can be obtained by appropriately irradiating ultraviolet laser light to an optical fiber that is sensitive to ultraviolet rays.
An earlier long period grating filter manufacturing apparatus comprises a high-output excimer laser optical source, a mirror, a lens, a silica mask, and an optical fiber. The optical source emits ultraviolet laser light. The mirror changes the path of laser light emitted by the optical source. The lens adjusts the focus of laser light whose path has been changed by the mirror. The silica mask selectively passes the laser light passed through the lens. The optical fiber has a core in which a long period grating is formed by being irradiated by the laser light passed through the silica mask to the optical fiber.
When the laser light passes through the lens and is irradiated upon the optical fiber contacting the silica mask, the refractive index of the optical fiber changes at regular periods, and the long period grating is formed on the optical fiber. Light is passed through the optical fiber using an optical source and is detected by a detector, and thus the optical characteristics of the long period grating filter are obtained.
In the long period grating filter manufacturing apparatus described above, the silica mask is comprised of chrome patterns obtained by coating and patterning chromium Cr on a silica substrate. The laser light is selectively passed by these chrome patterns. However, the chrome pattern has a damage threshold of 100 mJ/cm.sup.2, which makes it impossible to effectively use a high-output excimer laser light. Also, the silica mask is manufactured by forming the chrome patterns on the silica substrate, and thus has only one period which is determined by an initially designed pattern. Therefore, amplitude masks having different periods are required in order to obtain long period grating filters having different periods, thereby increasing manufacturing costs.